The present invention relates in general to the art of metallurgy, and more particularly to a method of fabricating structures of metallic glass.
Metallic glasses constitute a new class of materials whose amorphous structure produces excellent strength, hardness, ductility, corrosion resistance, wear properties, radiation stability, and isotropic mechanical behavior. Each of these glasses consists of a molten (or vaporized) alloy that has been chilled so abruptly (about 10.sup.6 .degree.K. per second) that it had no time to form crystals. The result is a homogeneous material completely free of the inclusions, dislocations, and grain boundaries that characterize ordinary metal specimens.
Amorphous metals are commercially available only in shapes having maximum surface area per unit mass of material, e.g., ribbons, sheets, wires, and powders. This is because rapid quenching from the melt can only be accomplished with a geometry that provides maximum heat transfer per unit mass of material.
Various procedures have been developed for providing rapid quenching by spreading the molten alloy in a thin layer against a metal substrate held at, or below, room temperature. The molten alloy is typically spread to a thickness of about 0.05 mm, which leads to a cooling rate of about 10.sup.6 .degree.C./sec. Details of the quenching process are given by R. Predecki, et al, in Trans. AIME 233, 1581 (1965), and also by R. C. Ruhl in Mat. Sci. & Eng., 1, 313 (1967). P. Duwez and R. H. Willens describe in Trans. AIME 227, 362 (1963) a gun technique in which a gaseous shock wave propels a drop of molten alloy against a copper substrate, the so-called "splat cooling" method. In Rev. Sci. Instr. 34, 445 (1963), P. Pietrokowsky describes a piston and anvil technique in which two metal plates come together rapidly to flatten and quench a drop of molten alloy falling between them. Ribbons or foils of amorphous metal can be produced by the casting technique described by R. Pond, Jr. and R. Maddin in Trans. Met. Soc. AIME 245, 2475 (1969) in which a molten metal stream impinges on the inner surface of a rapidly rotating hollow cylinder open at one end. Similarly, H. S. Chen and C. E. Miller describe in Rev. Sci. Inst. 41, 1237 (1970) a double rolls technique in which molten metal is squirted into the nip of a pair of rapidly rotating rollers.
The fact that amorphous metals are available only in thin ribbons or strips severely limits their practical utility. Commercial use would be much more widespread if these materials could be manufactured in standard structural shapes of appreciable thickness. Prior attempts to fabricate three-dimensional shapes have been unsuccessful, or only partially successful. The amorphous alloy may immediately crystallize during processing, and lose its desirable physical properties. The reason for this behavior is that the processing is carried out in a time-temperature regime that falls within the crystallization region for the alloy. Other attempts have shown that larger size pieces can be produced, but a process for reliably producing such larger size pieces while maintaining the desired properties has not been described.
The article, "Explosive Fabrication of Metallic Glasses", in Energy and Technology Review, October 1977, pp. ii and iii, indicated that a solid rod of metallic glass had been produced at the Lawrence Livermore Laboratory, Livermore, Calif., by packing powder into a steel pipe, immersing it in a liquid explosive, and detonating the explosive at one end of the pipe. This work was also reported in the following publications:
(1) C. F. Cline and R. W. Hopper, "Explosive Fabrication of Rapidly Quenched Materials", Scripta Metallurgica, Vol. 11, No. 12, pp. 1137-1138, 1977. PA1 (2) C. F. Cline, J. Mahler, M. Finger, W. Kuhl, and R. W. Hopper, "Explosive Fabrication of Rapidly Solidified Alloys", Proceedings of the Conference on "Rapid Solidification Processing: Principles and Technologies," Reston, Va., Nov. 13-16, 1977. PA1 D is the detonation velocity of the explosive charge; PA1 .alpha. is the ratio of active mass of explosive charge to active mass of the compacting material (amorphous material); and PA1 C is the specific heat of the compacting material.
The article, "Moulding of a Metallic Glass," in Mat. Res. Bull., Vol. 13, pp. 583-585, 1978, indicates that explosive forming has been explored in connection with amorphous metals and alloys. An amorphous alloy was pressed into a moulding at 390.degree. C. for up to one minute, and was still in an amorphous condition on the scale of a transmission electron microscope.
In U.S. Pat. No. 3,856,513 to Ho-Sou Chen, et al, patented Dec. 24, 1974, amorphous metals and amorphous metal articles are described. The compositions are quenched from the melt to the amorphous state. By the addition of certain elements, the alloys become better transformers, i.e., the amorphous state is more readily obtained, and is more thermally stable.
In U.S. Pat. No. 4,116,682 to D. E. Polk, patented Sept. 26, 1978, products of amorphous metal are described. The products may include cutting tools, such as razor blades. The alloys are rich in iron, nickel, cobalt, chromium, and/or manganese. The alloys contain at least one element from each of the three groups of elements, and are low in metalloids compared to previously known, liquid-quenched, amorphous alloys. A class of amorphous metal compositions are describe which are readily quenched to the amorphous state, in which they display improved physical characteristics. The class of compositions is defined by the formula N.sub.a T.sub.b X.sub.c, where N is any combination of elements from the group consisting of iron, nickel, cobalt, chromium, and manganese; T is any combination of elements from the group consisting of zirconium, tantalum, niobium, molybdenum, tungsten, yttrium, titanium, and vanadium; and X is any combination of elements in the group consisting of boron, silicon, phosphorous, carbon, germanium, and arsenic.
In U.S. Pat. No. 3,022,544 to D. L. Coursen, et al, patented Feb. 27, 1962, the explosive compaction of powders is described. A tubular container is surrounded by a mass of powder, with a layer of high velocity detonating explosive. The explosive layer has a substantially continuous and uniform composition. A detonating explosive having a conical configuration is positioned with the base of the cone adjacent to one edge of said layer of explosive. The conical explosive is initiated at its apex to form a compact by compressing the powder within the container.
Some experimentation has been reported indicating that larger size pieces can be produced by pressing metallic glass powder into larger size pieces. Such pressing operations, if not properly performed, could destroy the desirable properties of the metallic glass material and result in the larger size pieces lacking the desirable properties or with the desirable properties being reduced. A process that would produce larger size pieces and insure that the larger size pieces retain the desirable properties of the metallic glass would be a significant advancement of the art.